Petrographical Differentiation of certain dykes. 163 
part marginal. Xo quartz was observed. At six feet the struct- 
ure is again ophitic. Quartz is present. Magnetite is in large 
sized, sparsely scattered grains. A little pyrite also occurs. The 
structure of the middle part of the dyke is for the most part 
granular but with some idiomorphic plagioclase. The augite is 
generally fresh, but has associated with it hornblende and chlorite 
as alteration products. Quartz is abundant with uniform orienta- 
tion over wide, and in the section isolated, areas. The following 
are the figures for the silica percentage and specific gravity of the 
three specimens : 
Contact. 
Six feet from 
contact. 
Middle. 
Si0 2 
Sp. g. 
48.65 
3.088 
47.92 
3.079 
49.28 
3.016 
Similar specimens were taken from a dyke on Risk}- Island, 
Rainy Lake, which is nearly in a line with the Stop Island dyke and 
the Shoe Bay dyke last described. The contact rock is as before 
distinctly a porphyrite similar to that represented in fig. I. The 
minerals are all fresh. Plagioclase in large crystals and augite in 
single and in polysomatic grains, are imbedded in the usual fine 
ground mass, which in this, and in all the dykes described, is ap- 
parently holocrystalline and micro-ophitic. At six feet from the 
dyke wall the structure is ophitic but with some allotriomorphic 
plagioclase. The augite is niostby replaced b}- hornblende. Quartz 
is plentiful in pegmatitic development. Magnetite or titanic iron 
is abundant but without any trace of leucoxene. In the middle 
of the dyke the ophitic structure is still observable but most of 
the constituent minerals are allotriomorphic. The augite is very 
largely replaced by hornblende. Quartz is observed to present 
the same ophitic relations to the idiomorphic plagioclase as does 
the augite, and it contains inclusions of apatite and microlites. 
Leucoxene with cleavage traces of titanic iron is abundant and in 
large grains. The following are the figures for the percentage of 
silica and specific gravity of these specimens : 
Contact. 
Six feet from 
contact. 
Middle. 
Si0 2 
Sp. g. 
49.77 
3.100 
49.64 
3.044 
52.31 
3.016 
